Horizontally divided, the top stripe dark green with a centred (see below) yellow castle with red gate and windows, as high as 4/9 of the flag's height, the bottom stripe vertically divided into nine yellow-red stripes. Ratio 2:3.

The flag is a banner of the arms (blazonedper fess Vert a castle Or gate and windows Gules, and Or four pallets Gules). The castle is that of Cubelles and the yellow-red stripes remember the former royal jurisdiction over the municipality. Flag adopted 17th January 2000. Source for image and text: Fluvià 2001.

The official description says the castle is "separated from the top and bottom sides of the top stripe by 1/36 of the flag's height" – how much simple it would be to say "centred"...

Some additional information, extracted from this webpage of the Statistics Institute of Catalonia (Idescat):